In total knee arthroplasty procedures, a number of instruments and techniques can be utilized. In many procedures, the surgery begins with exposing the ends of the bones that make up the knee, including the femur, the tibia, and the patella. An elongated drill hole is then made in the distal femur, and an intramedullary rod is inserted into the hole. A first cutting guide is then inserted onto this rod and positioned in the correct anteroposterior and rotational orientation, and then the anterior cortex is cut with a saw. The first cutting guide is then removed and replaced with a second cutting guide that is used to cut the distal femur in the correct valgus angle. The second cutting guide is then removed, along with the intramedullary rod. The femur can then be measured to determine the size of the femoral component that will be used.
After sizing the femoral component, yet another cutting guide (i.e., a third cutting guide) is positioned on the cut surfaces and pinned or secured in place. Anterior, posterior, and chamfer cuts are made using a saw as guided by the third cutting guide, and then the third cutting guide is removed. A fourth cutting guide is then positioned and pinned on the cut surfaces to make a cut know as the “box cut”. After this cut is done, the fourth cutting guide is removed. A trial femoral component is then positioned relative to the cut surfaces to check for proper fit relative to the bone.
The tibia is then subluxed forward so that an extramedullary guide can be positioned relative to it. In particular, this extramedullary guide is positioned so that it can attach from the tibial plateau on its proximal part to the ankle at its distal part. At this point, a fifth cutting guide can be positioned relative to the tibia and pinned or secured in place. Alternatively, a hole can be drilled into the tibia in a similar manner to the technique used to drill into the femur so that another intramedullary rod can be inserted. The fifth cutting guide can then be positioned on this rod. In either case, once the fifth cutting guide is in its desired location, the tibia can be cut and the guide can be removed. The proximal tibia is generally prepared using a short punch that will make a tubular hole in the bone, and a stem of a trial tibial component can then be inserted into the tubular hole.
Once the trial femoral and tibial components are placed in their desired locations, the relationship between the femur and tibia is tested and corrected so that they are balanced in the medial and lateral sides during extension and flexion. The whole lower extremity axis is also checked to be sure that it is straight. Tight structures and/or incorrect bone cuts may make the system unbalanced and/or make the axis incorrectly aligned, wherein either or both of these issues will require correction. In particular, if the structures are too tight, they can be released. If the bone cuts made were too small or thin, they can be cut again using the previous sequence of the use of cutting guides and cuts. If the bone cuts are too large or thick, appropriate inserts can be added to the bone to make it the desired size and shape. All of these corrections or adjustments require extra steps that are inconvenient and may ultimately result in a less successful surgical outcome.
As set out above, the successful outcome of a total knee arthroplasty includes achieving accurate bone cuts and adequate ligament balancing. In particular, the bone cuts must be accurately made in relation to the mechanical axis of the femur, which can be difficult to identify because it extends from the center of the femoral head to the center of the knee joint, wherein the femoral head cannot be visualized during total knee surgery. The most widely used method of locating the mechanical axis is with a rod that is positioned in the femoral medullary canal. The mechanical axis is then estimated to be positioned approximately 6 degrees medially from the axis of the rod. Although this method can be easy to implement, it is not necessarily accurate due to variations in the anatomy of the femur and due to the play between the rod and the medullary canal in which it is positioned. This method also cannot determine the direction of the mechanical axis when viewed in the sagittal plane. Furthermore, this method requires the medullary canal to be violated, which can potentially lead to undesirable blood loss and possible complications. Violating the canal can also potentially lead to fat embolism or activation of coagulation. Another way of locating the mechanical axis is by using computerized navigation equipment to identify bony landmarks and relate them to the motion of the femur to locate the mechanical axis. However, such equipment is often relatively expensive, and can be cumbersome to use in surgery.
After the femur and tibia are balanced and their alignment is straight, the patella can be resurfaced and/or the knee can be moved through its range of motion to check for correct patella tracking. In cases where the patella is going to be resurfaced, the patella is everted when the knee is straight. The patellar thickness can be measured, such as with a caliper, and then a patella cutting guide is applied to the patella to cut the amount of patella that will later be replaced by plastic. Once the patella is cut, a trial plastic component can be positioned relative to the cut area and the whole knee is then checked for balance, alignment, and patellar movement or tracking. After this process is complete, the trial components are removed, the bone ends are cleaned and dried, bone cement is applied in the appropriate locations, and the final components are placed in their desired locations. Balance, alignment, and patellar tracking are checked again, the knee is closed, and the surgery is considered to be complete.
Although many of the described processes work reasonably well, there is a need to provide systems and methods for use in total knee arthroplasty that are less invasive, more accurate, and more simple, both for locating the femoral mechanical axis accurately in both the coronal and sagittal planes and for making accurate bone cuts using more simplified cutting guides.